GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderPelosi aide hopeful White House will support drug-pricing bill despite criticism Overnight Energy: BLM staff face choice of relocation or resignation as agency moves | Trump says he's 'very much into climate' | EPA rule would expand limits on scientific studies Juan Williams: Republicans flee Trump MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonImpeachment hearings don't move needle with Senate GOP GOP divided over impeachment trial strategy 'Too Far Left' hashtag trends on Twitter MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchTrump awards Medal of Freedom to racing industry icon Roger Penske Trump holds more Medal of Freedom ceremonies than predecessors but awards fewer medals Trump to award Medal of Freedom to former Attorney General Edwin Meese MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerLindsey Graham basks in the impeachment spotlight The Hill's 12:30 Report — Presented by Nareit — White House cheers Republicans for storming impeachment hearing GOP senators frustrated with Romney jabs at Trump MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellLawmakers skeptical of progress on spending deal as wall battle looms Impeachment battle looms over must-pass defense bill 'Saturday Night Live' presents Trump impeachment hearings with 'pizzazz' of soap opera MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerOvernight Health Care: Trump officials making changes to drug pricing proposal | House panel advances flavored e-cig ban | Senators press FDA tobacco chief on vaping ban Chad Wolf becomes acting DHS secretary Schumer blocks drug pricing measure during Senate fight, seeking larger action MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurraySenators press FDA tobacco chief on status of vaping ban Retirement bill blocked in Senate amid fight over amendments Senate Democrats call on White House to abandon plan to collect DNA from migrants MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeyHillicon Valley: Twitter shares more details on political ad rules | Supreme Court takes up Google-Oracle fight | Pentagon chief defends Microsoft cloud contract House, Senate announce agreement on anti-robocall bill Democratic senators introduce bill to block funding for border wall live stream MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenHonoring service before self To fight Mexican drug cartels, we must designate them Foreign Terrorist Organizations Bannon returns from exile to wage impeachment battle for Trump MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpTrump reversed course on flavored e-cigarette ban over fear of job losses: report Trump to award National Medal of Arts to actor Jon Voight Sondland notified Trump officials of investigation push ahead of Ukraine call: report MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnTrump circuit court nominee in jeopardy amid GOP opposition Progressive freshmen jump into leadership PAC fundraising On The Money: US paid record .1B in tariffs in September | Dems ramp up oversight of 'opportunity zones' | Judge hints at letting House lawsuit over Trump tax returns proceed MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.